The present invention relates to a method for manufacturing a semiconductor device and, more specifically, it relates to a method for depositing phosphosilicate glass films on the surfaces of substrates (e.g. silicon substrates or aluminum wire patterns on silicon substrates) for semiconductor devices.
As is known in the art, a phosphosilicate glass film is widely used as a protective coating (or a passivation film), an interlaminar insulating film of multi-wire pattern layers or a solid diffusing source for an N-type impurity. In order to chemically deposit phosphosilicate glass films on substrates, a so-called "atmospheric pressure chemical vapor deposition method" is conventionally used. This is due to the fact that phosphosilicate glass films having a uniform thickness and having a uniform phosphorous concentration, can be readily obtained. In the atmospheric pressure method, a phosphosilicate glass film is deposited on a substrate in, for example, a bell-jar type reaction apparatus from a reaction gas mixture including monosilane (SiH.sub.4), phosphine (PH.sub.3) and oxygen (O.sub.2) under atmospheric pressure and at a temperature of 350.degree. through 450.degree. C. However, the atmospheric pressure chemical vapor deposition methods have disadvantages in that mass production of the deposited phosphosilicate glass films on the substrates cannot be achieved, and cracks are likely to be generated in the deposited films during, for example, an annealing treatment at approximately 450.degree. C. (i.e. to remove the surface state formed in the semiconductor device after the deposition of the phosphosilicate glass films). The generation of the cracks results in the decrease in the reliability of the semiconductor device.
In addition, a so-called "low pressure chemical vapor deposition" is also used. In this method, a phosphosilicate glass film is deposited on a substrate from a reaction gas mixture including SiH.sub.4, PH.sub.3 and O.sub.2 under a low pressure (e.g. 1 Torr or less) and at a temperature of 350.degree. through 450.degree. C.
For instance, as shown in FIGS. 1 and 2, a reaction tube 10 made of, for example, quartz having an inner diameter of 120 mm.phi. and a uniform heating zone length of approximately 100 mm, is used for this purpose. In the reaction tube 10, gas inlets 11 and 12 are attached for feeding a gas mixture of SiH.sub.4 and PH.sub.3 and O.sub.2 to one end thereof and an exhaust outlet 13 is attached to the other end thereof. Around the reaction tube 10, a heater 14 is mounted. In the practice of the chemical vapor deposition, plural substrates 15 for semiconductor devices made of, for example, silicon (i.e. wafers) are placed on a wafer holder 16 in such a state that the main surfaces of the wafers 15 are substantially vertically aligned with respect to one another and substantially perpendicularly intersect the central axis of the reaction tube 10. Then, a gas mixture of SiH.sub.4 and PH.sub.3 and an O.sub.2 gas are fed from the gas inlets 11 and 12, while the inside of the reaction tube 10 is evacuated from the exhaust outlet 13 by means of a vacuum pump (not shown). Thus, phosphosilicate glass films are formed on the main surfaces of the wafers 15 under the conditions of, for example, a pressure of 0.5-1.0 Torr and a temperature of approximately 425.degree. C. for 100 minutes.
However, there are problems in these conventional low pressure chemical vapor deposition methods in that when the amount of the O.sub.2 gas fed into the reaction tube 10 is increased, the thicknesses and the qualities of the phosphosilicate glass films deposited on the surfaces of the plural wafers 15 are not uniform. For instance, the film thickness distribution of the phosphosilicate glass films deposited on the surfaces of the plural wafers is as much as .+-.30%. For this reason, the phosphosilicate glass films are conventionally deposited at a mol ratio of O.sub.2 /SiH.sub.4 in the reaction feed gas of less than 2 (i.e. approximately 0.5-1.5). However, according to this method, hydrogen remains and is included in the deposited phosphosilicate glass films due to the fact that the amount of the O.sub.2 gas in the reaction gas mixture is not sufficient. In the case where the phosphosilicate glass film contains a relatively large amount of hydrogen, cracks are likely to be generated in the deposited phosphosilicate glass films. The formation of the cracks in the deposited phosphosilicate glass films makes the phosphosilicate glass films useless. In addition, a relatively large amount of hydrogen causes the accumulation of phosphorus at the interface of the substrate and the phosphosilicate glass film when the phosphorous accumulated phosphosilicate glass films are heat-treated at a temperature of 900.degree. C. or more.